Thanks to an advantage that secondary copper loss of a rotor is not produced in contrast with an induction motor, the reluctance motor attracts considerable attention as a driving motor for an electric vehicle, a machine tool or the like. However, this reluctance motor generally has poor power-factor and thus, requires improvement of the construction of the rotor core, a driving method, etc. in order to be used for industrial purposes. In recent years, a technology for improving the power-factor by providing flux barriers in a plurality of rows on a core sheet of the rotor core has been developed as described in a paper entitled Development of Multi-Flux Reluctance by Yukio Honda et al. in Proceedings No. 1029 published on Mar. 10, 1996 for a national meeting 1996 of the Electrical Society of Japan.
FIGS. 31 to 33 show an example of a construction of a rotor core of this improved known reluctance motor. In FIG. 31, a plurality of arcuate flux barriers 162 are provided on a circular core sheet 161 formed from an electromagnetic steel plate so as to convexly confront an axis 163 of the core sheet 161. Each of the flux barriers 162 comprises a through-slit of about 1 mm in width, and is formed by a press. In order to impart strength to the core sheet 161, against centrifugal force applied to the core sheet 161 during its rotation, an outer peripheral rim 164 having a predetermined width is provided at an outer periphery of the core sheet 161.
By laminating several tens of the core sheets 161 on one another on a rotor shaft 165, a rotor core 166 is obtained as shown in FIG. 32. If this rotor core 166 is set in a stator 167 as shown in FIG. 33, a rotational magnetic field is given to the rotor core 166 by a plurality of field portions 168 of the stator 167 and thus, a reluctance torque T is produced. This reluctance torque T is expressed by the following formula (1). EQU T=Pn (Ld-Lq) id.times.iq (1)
In the above formula (1), "Pn" denotes the number of pairs of poles, "Ld" denotes a direct-axis inductance, "Lq" denotes a quadrature-axis inductance, "id" denotes a direct-axis current and "iq" denotes a quadrature-axis current. It is seen from the above formula (1) that performance of the reluctance motor relies on magnitude of (Ld-Lq). In order to increase (Ld-Lq), it has been a general practice that the above mentioned flux barriers 162 formed by the slits are provided on the core sheet 161 so as to impart resistance to a quadrature-axis magnetic path traversing the slits, while a direct-axis magnetic path interposed between the slits is secured.
In the above construction of the known rotor core 166, the slits each having a width of about 1 mm are formed on the core sheet 161 by a press, and a strip is provided between neighboring ones of the slits such that the strips are coupled with each other at a predetermined width by the outer peripheral rim 164.
However, in this construction of the known rotor core 166, since the quadrature-axis magnetic flux penetrates each slit, the value of the quadrature-axis inductance Lq is increased and thus, the reluctance torque T decreases. On the contrary, if the width of each slit is increased so as to lessen the quadrature-axis magnetic flux, the width of each strip is also reduced, so that the value of the direct-axis inductance Ld is reduced and thus, the value of the reluctance torque T also decreases.
Meanwhile, in the construction of the known rotor core 166, if the number of revolutions of the motor is increased, stress concentration may result, via centrifugal force in the vicinity of radially inner slits of the core sheet 161, especially at the outer peripheral rim 164 at a radially innermost slit of the core sheet 161. This possibly results in deformation of the rotor core 166.
Large stress is applied to the outer peripheral rim 164 at the radially inner slits of the core sheet 161 for the following reason. The radially outer strips of the core sheet 161, which are supported by the outer peripheral rim 164, are short in length and thus, are light in weight. However, the radially inner strips of the core sheet 161, which are supported by the outer peripheral rim 164, become gradually larger in length and thus, become gradually heavier in weight. Therefore, centrifugal force produced by rotation of the rotor core 166 becomes gradually larger towards the radially innermost slit of the core sheet 161 along the outer peripheral rim 164. Furthermore, by driving the rotor core 166 for its rotation, the strips projecting towards the center of the rotor core 166 are urged out of the rotor core 166. As a result, the strips projecting towards the center of the rotor core 166 would depress the outer peripheral rim 164 outwardly so as to project out of the rotor core 166. At this time, the strips become larger in size towards the radially innermost slit of the core sheet 161 along the outer peripheral rim 164 and therefore, produce larger force for depressing the outer peripheral rim 164 outwardly. Therefore, as location on the core sheet 161 approaches the stress concentration portions on the outer peripheral rim 164 at the radially innermost slit of the core sheet 161, force for deforming the rotor core 166 becomes extraordinarily larger.
Hence, if width of the outer peripheral rim 164 is increased so as to prevent deformation of the rotor core 166 even at the time of high-speed rotation of the rotor core 166, the outer peripheral rim 164 coupling the strips with each other is not subjected to magnetic saturation. Therefore, since quadrature-axis magnetic flux leaks through the outer peripheral rim 164, the quadrature-axis inductance Lq becomes large and thus, the rotor core 166 cannot be driven for its rotation efficiently.